1. FIELD OF THE INVENTION
The present invention relates to biodegradable material, suitable for use in such things as wound dressings as well as other applications.
2. DESCRIPTION OF THE RELATED ART
Biodegradable polymers have become increasingly important for a variety of biomedical applications including tissue engineering scaffolds. However, relatively few biodegradable, particularly elastomeric, polymers have been developed which are presently in use.
Polymeric devices which intentionally degrade and disappear upon completion of their function may mitigate the inevitable, usually negative physiologic responses (eg. fibrous encapsulation) which may limit long-term device success. Thus, an array of degradable polymers have been developed and studied for many uses..sup.4,5,6 However, relatively few of these degradable materials are elastomeric polymers. Rather, the majority of degradable polymers are essentially hard, brittle materials,.sup.7 developed for drug delivery uses. With the increasing interest in tissue engineering degradable materials exhibiting a wide variety of physical properties are necessary to integrate with the various tissues of the body.
Segmented polyurethane elastomers have enjoyed wide use as biomaterials due to their excellent mechanical properties and great chemical versatility..sup.8 The vast majority of research devoted to the development of biomedical polyurethanes has focused on long-term applications such as vascular grafts and pacemaker lead insulators..sup.8 Accordingly, a significant amount of research involving the degradation of non-degradable polyurethanes has been undertaken..sup.9-12 The research here indicates that the urethane, urea and ester groups which may be present in the polyurethane are susceptible to limited hydrolysis (both chemical and enzymatic) in biological media. In addition, ether groups often present in the soft segment are susceptible to oxidative degradation via phagocyte-derived oxidants,.sup.13 and this is believed to be a key component of the stress cracking phenomenon noted for pacemaker lead insulation.
Despite the progress thus far in the development of polyurethanes, relatively little research has been directed at developing intentionally degradable polyurethanes for temporary implantation. Several papers were published in the early 1980's describing polyurethane/polylactide blends as degradable materials for skin substitutes, vascular prostheses and nerve regeneration guides..sup.14,15 However, in these cases the polyurethane portion of the blend was non-degradable and served only to provide favorable mechanical properties. Subsequent work by Bruin et al.sup.16 involved the synthesis of crosslinked polyurethane networks incorporating lactide or glycolide and .epsilon.-caprolactone joined by a lysine-based diisocyanate. These polymers displayed good elastomeric properties and were found to degrade within 26 weeks in vitro and 12 weeks in vivo (subcutaneous implantation in guinea pigs). However, a drawback of this approach is that the highly crosslinked polymer may not be processed by standard techniques such as solution casting or melt processing as is the case for typical linear, segmented polyurethanes. Cohn et al developed a series of elastomeric polyester-polyether-polyurethane block copolymers intended for use as surgical articles..sup.17 More recently, poly(phosphoester urethanes) have been synthesized by Dahiyat et al.sup.18 for drug delivery applications. However, these polymers are relatively stiff, low tensile strength materials, which may preclude their use as elastomeric biomaterials.
It is therefore an object of the present invention to provide novel materials for use in wound dressings and other tissue engineering applications.
It is a further object of the present to provide novel polyurethanes which are biodegradable.
It is a further object of the present invention to provide novel polyurethanes of the segmented variety which are biodegradable by enzyme-mediated hydrolysis.
It is a further object of the present invention to provide a chain extender for use in the formation of biodegradable polyurethanes.